pristine-xz: Multiple fixes for multi-threading and pixz support

Kevin Lockerequested to merge
kevinoid/pristine-tar:multithreading-and-pixz into master

This pull request improves the detection of files created with multi-threading, adds support for pixz, and switches to -T0 for maximal performance. In my testing this fixed all open issues against pristine-xz, except #851291. If you'd like more explanation about any of the changes or if you'd prefer I split these up into separate pull requests, let me know.

Thanks for considering, Kevin

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